[QUOTE=LHR/MEL/Europe FF;24486719]I don't believe this was a denied boarding for the purposes of EU261, which requires the passenger to have a confirmed reservation.
UA cancelled the reservation (separate issue to be pursued).
The $100 appears to be a 'goodwill' recognition of inconvenience, rather than an admission of anything.
I understand your doubts, but the EU261/2004 legal ratio is to protect the consumers against airlines bad practices.
Well, I got the point you are highlighting, but denied boarding could mean also when you are denied to board because the airline, suddenly, cancels your tkt, calling down some odd reasons. Otherwise the airlines could cancel the tkt few minutes before your airport check-in, pretending to demonstrate that you don't have a valid/existing tkt.
In his case, I would proceed. It's costless also.